hey Joe, i've been running GT KT66HP's in my early 66, decided to try something different and got some Shugaung's from LV.....seem like a great tube, newer production but they are not made anymore as the factory closed.....I'm not sure but these may be the same tubes that KCA sells under the Valve Arts name...... I'm running old GZ34's but i may need a fresher one, one runs the plate voltage at 425 (it is Very used) and the other at 450 volts, i've got a newer Russian one that hits @ 475 which Z recommends as a good plate voltage.....I may try a new JJ.... I'm digging an old Mullard pre, have a Telefunken which is nice almost to nice, very hi-fi sound....I'd love to score some GEC KT66's some day, gotta find a good deal or Spend!!! on those....
the Dario is a european made EF86 (holland i beleive), very much like the amperex. Personally any of the Euro NOS EF86 brands sound good and are quite reliable.. the reliability is their strongest quality IMO.
I loaded up my 66 with Genelex (GEC) brown base clear bottle KT 66's, a British military GEC EF-86 and I scored a steel based Mullard GZ-34 (1954) for the rectifier. I wanted the amp to be at its best for the album I recorded with it otherwise I probably would have left the stock tubes in as the amp sounded fantastic out of the box. The GEC's are pretty special though.
I just stumbled upon this in the gear forum....this might help all of us...
Here is what I saved from Z's post at least a couple years ago:
"Since I have stuffed approx. 400 RT.66's with tubes since it's release in 1998, I do have a bit of experience on the subject _Joker, and I'll try to answer your question.
Ok, looking from the rear of the RT 66 going left to right:
V1= EF-86, you will be hard pressed to greatly improve over a good Svetlana. Eric Barber and myself collaborated on EF-86 designs while he was with Svetlana, and some of the designs were posted on their site.
But being guitar players we must try to change/improve anything that generates tone.
So for V1 there are many imported EF-86's that will change the RT.66's front-end response.
Mullard shiny shield, very nice tube, slightly less gain, very top-endy in a good way. Works well with Humbuckers, but watch out with some Tele's and G&l's.
Seimens gunmetal gray shield , more gain, and a very warm tone, great with strat's, but can get muddy with some not all DARK Les Pauls.
Mazda gunmetal gray shield, close to Seimens, just not as bassey, more mid's.
Tesla made a EF-86, geat sounding, BUT very prone to microphonics.
Telefunkin low gain rich tone, but a bit HiFi, high dollar.
Amperex World Logo, low gain and very bland tone to my ears.
Now V2- I use a well balanced Sovtek 12AX7 LPS, full power, and full bandwidth.
Ei 12AX7/ECC 83 , if you can find a good one, that is a NOS dark plate, that's not microphonic ,they sound great. David Grissom uses a Ei in his RT.66.
Phillips 12AX7 WA large plate, good drive and a very warm tone.
RCA/GE both fine, slight loss of power, but nice hollow mid tone, and nice top end sparkle.
Tele's/Mullards/Brimmars, all three different but close to the same , balanced and smooth, slight loss of hair at full drive.
Ok V3 & V4 - I currently ship with version 3 Groove Tube Russian KT-66-HP. Version 1 was fine, version 2 had shorting/reliability problems. Fixed in Ver 3 as well as upping the power , hence HP or High Performance.
The Sino KT-66 is a very nice slightly lower powered tube in the RT.66. It does sound more like the GEC KT-66 then GT's. Reliability problems batch to batch, like halo getters falling loose inside the tube.
Finally V5- To get the full 40 watts at clip you must use a New Sovtek or Mullard GZ-34. If you have an older RT.66 it may have a Chinese 5AR4/GZ-34 Those were the only ones being built in 1998, replace it before it goes nuclear like they all will in less then 5 years of use.
But if you want to change the overall response of the RT.66 try a Sylvania green labeled 5U4, or a GE 5V4. Remember to adjust bias, as both of these tubes will lower plate voltage at idle, and more so at clip. Set the KT-66's from 28 to 34 mA per tube and you'll then hear the difference the rectifier makes. It will increase sustain and compression , lower overall power, and will feel spongier , which might be a good thing for you. I personally like the GZ-34 in my RT.66."
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